1. Field of the Invention
The present invention is directed to a battery charger for charging a battery from a dc power source, and more particularly to a battery charger incorporating a DC-to-DC voltage converter for charging a battery from a dc voltage power source that may have a terminal voltage equal to or less than the nominal rated voltage to be charged.
2. Description of the Prior Art
As a result of the extensive use of portable apparatus powered by an incorporated rechargeable battery, there have been increased demands for charging the battery from a dc power source such as an automobile battery readily available at outdoor locations. However, in recent years, it is not uncommon to power the apparatus by a battery having the nominal rated voltage equal to or greater than the terminal voltage of the automobile battery. To this end, there have been proposed a prior battery charger utilizing a step-up transformer for charging a battery from a lower dc voltage source, for example, as shown in U.S. Pat. No. 3,594,627. The step-up transformer employed in the prior battery charger has a primary winding coupled to the dc voltage source through a switching transistor, a secondary winding coupled to the battery to be charged, and a feedback winding connected in circuit with the switching transistor. The switching transistor is controlled by a suitable oscillation circuit to be alternately turned on and off for developing an induced voltage across the secondary winding of the step-up transformer.
A typical circuit arrangement of the prior battery charger is exemplarily shown in FIG. 8 of the attached drawings for easy understanding of the operation of the prior art charger. In the circuit of FIG. 8, a dc voltage source S is coupled to a battery B to be charged through a step-up transformer T having a primary winding L1, a secondary winding L2, and a feedback winding L3. The primary winding L1 receives the current from the dc voltage source S by the operation of a switching transistor Q. The switching transistor Q is connected in circuit with a starting resistor R1, resistor R2, capacitor C, and the feedback winding L3 to form a blocking oscillator which operates to alternately turn on and off the transistor Q at a suitable frequency. The blocking oscillator supplies an intermittent current to the primary winding L1 of the step-up transformer T which responds to induce an increased voltage across its secondary winding L2. The resulting ac voltage is then rectified by a half-wave rectifier diode D to be applied across the battery B for flowing the charge current to the battery B. However, the circuit arrangement of the above charger has an inherent drawback of amplifying the energy loss significantly with the increase of induced voltage required for charging. This is easily understood from the following equation representing the energy loss Ws in the step-up transformer of the above circuit. EQU Ws=(1-.eta.)E.sub.0 I.sub.0 =(1-.eta.)E.sub.1 I.sub.1 /.eta.,
where E.sub.0 is the terminal voltage of the dc power source S, I.sub.0 is the current flowing into the primary winding L1, E.sub.1 is the voltage induced across the secondary winding L2, I.sub.1 is the charge current to the battery B, and .eta. is the transformation efficiency. That is, it is apparent from the above equation that the energy loss Ws is in direct proportion to the induced voltage E.sub.1 required. Further, the increase of the induced voltage will require the transformer to be of larger capacity and therefore more bulky and expensive.